In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. User equipment units (UEs) may be, for example, mobile telephones (“cellular” telephones), desktop computers, laptop computers, and tablet computers, or stationary units, with wireless communication capability to communicate voice and/or data with a radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called “NodeB” or (in Long Term Evolution (LTE)) eNodeB (eNB). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the UEs within range of the base stations.
Specifications for an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are on-going within the 3rd Generation Partnership Project (3GPP). Another name used for E-UTRAN is the Long Term Evolution (LTE) Radio Access Network (RAN). LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected directly to a core network rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller node are performed by the radio base stations nodes. As such, the radio access network of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller nodes.
The evolved UTRAN comprises evolved base station nodes, e.g., evolved NodeBs or eNBs, providing user-plane and control-plane protocol terminations toward the UEs. The eNB hosts the PHYsical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. The eNodeB also offers Radio Resource Control (RRC) functionality corresponding to the control plane. The eNodeB performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated UL QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink (DL)/uplink (UL) user plane packet headers.
The LTE standard is based on multi-carrier based radio access schemes, Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and Single-Carrier Frequency-Division Multiple Access (SC-FDMA) in the uplink. Orthogonal FDM's (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion.
In the time domain, one subframe, Transmission Time Interval (TTI), of 1 ms duration is divided into 12 or 14 OFDM (or SC-FDMA) symbols, depending on the configuration. One OFDM (or SC-FDMA) symbol includes a number of sub-carriers in the frequency domain, depending on the channel bandwidth and configuration. One sub-carrier on one OFDM (or SC-FDMA) symbol is referred to as a resource element (RE). See, e.g., 3GPP Technical Specification 36.211.
In LTE, no dedicated data channels are used; instead, shared channel resources are used in both downlink and uplink. These shared resources, the Physical Downlink Shared Channel (PDSCH) and the Physical Uplink Shared Channel (PUSCH), are each controlled by one or more schedulers that assign(s) different parts of the downlink and uplink shared channels to different UEs for reception and transmission, respectively.
The downlink assignments (sometimes called downlink grants) for the Physical Downlink Shared Channel (PDSCH) and uplink grants for the Physical Uplink Shared Channel (PUSCH) are transmitted to UEs in a control region covering a few OFDM symbols in the beginning of each downlink subframe. The Physical Downlink Shared Channel (PDSCH) is transmitted in a data region covering all or a subset of the OFDM symbols in each downlink subframe. The size of the control region may be either, one, two, three or four OFDM symbols, and is set dynamically per subframe, sometimes within semi-statically configured restrictions (e.g., Relay-Physical Downlink Control Channel (R-PDCCH), cross-scheduled UEs in carrier aggregation).
Each assignment PDSCH or PUSCH is transmitted as a message on a physical channel named the Physical Downlink Control Channel (PDCCH) in the control region. There are typically multiple Physical Downlink Control Channels (PDCCHs) in each subframe. Downlink assignments and uplink grants are defined for only one transmission time interval (TTI). Thus, a new downlink assignment or uplink grant is sent for each TTI where the UE is expected to receive transmission, except for semi-persistent scheduling where scheduling is performed for a defined number of TTIs by identifying that a downlink assignment or uplink grant is valid for a one TTI at a time, reoccurring with a configured periodicity until it is released by a defined PDCCH message or by RRC signalling.
A PDCCH is mapped to (e.g., comprises) a number of control channel elements (CCEs). Each CCE consists of thirty six Resource Elements (REs).
A PDCCH can be transmitted with quadrature phase-sift keying (QPSK) modulation and channel coding, and can include an aggregation level of 1, 2, 4 or 8 CCEs, See, e.g., 3GPP Technical Specification 36.213. Each control channel element (CCE) may only be utilized on one aggregation level at the time. The total number of available control channel element (CCEs) in a subframe will vary depending on several parameters like number of OFDM symbols used for PDCCH, number of antennas used for transmission/reception, system bandwidth, Physical HARQ Indicator Channel (PHICH) size, etc.
The number of CCEs, and thereby the code rate, used for transmission of a PDCCH message from a network node to a UE can be controlled based on channel state information (CSI) that is reported by the UE. The CSI can include a Channel-Quality Indication (CQI), a rank indication, and a precoder matrix indication. A UE generates the CSI based on measurements performed on CSI reference signals (RS) transmitted by the network node. The interference measured on these references signals, from RSs or data traffic, might be correlated with the interference experienced by a PDCCH. However, data traffic and its resulting interference dynamically changes over time, and these changes may not be correlated with PDCCH interference changes.
Consequently, controlling CCE allocation (a type of Link Adaptation (LA)) for PDCCH messages solely based on CSI may lead to inefficient allocation of CCEs. Inefficient allocation of CCEs may be particularly problematic when handling low-bandwidth services and/or uplink communications (which are typically limited to more narrowband allocation than downlink due to UE transmission power limitations) where a lack of available CCEs can limit how many UEs can be scheduled in the same TTI on different frequency segments to utilize the available bandwidth.
In a heterogeneous network deployment, the base stations (eNB) typically transmit with different power levels. This leads to imbalance problems around low power nodes since the high-power node (macro base station) may be selected as the serving cell since received signal strength is higher, although the pathloss to the low-power node (micro base station) is lower. To offload the macro (high-power) node and also improve the uplink (UL) performance, cell-section offset, also known as cell Range Expansion, can be used.
When the range of the micro (low-power) node is extended by RE, the UEs in the range expansion zone are heavily interfered by the macro node. This interference can be mitigated according to 3GPP release 10 using so called Almost Blank Subframes (ABS), see 3GPP Technical Specification 36.300, where certain subframes are protected, meaning that the macro node is not allowed to transmit in those subframes (as a result, the subframes are almost blank). ABS can be seen as a special case of Reduced Power Subframes (RPS), where the macro node is allowed to transmit, but with reduced power, in the protected subframes.
In a network where RPS is used, a wireless unit (or UE) may thus experience different interference levels for the PDCCH depending on whether the subframe is a reduced power subframe or not, i.e. a regular power subframe. Consequently, the link adaptation of the PDCCH may not be optimal for both reduced power subframes and regular power subframes.